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AP-19

In some electronics applications, a particular bias voltage may have to
range from a positive voltage to a
negative voltage or vice versa. The conventional solution is to utilize
a high-voltage relay to reverse the
polarity. However, this approach introduces a mechanical element to the
system, which dramatically reduces
the MTBF of the HVPS sub-system. This application note is presented as
an illustration of a simple solution
for low-current and electrostatic-bias applications. For higher current
applications, contact UltraVolt's customer
service department.

The simplest form of electronic polarity reversing
is accomplished by using a positive supply and a negative
supply, joined together through a pair of series resistors:

This configuration provides seamless control from 1/2 of
the positive supply's maximum output voltage to 0 VDC
to 1/2 of the negative supply's maximum output voltage. For example, an
UltraVolt 4A12-P4 and 4A12-N4
combination would be adjustable from +2kV to -2kV. The supplies could be
of the same voltage capability or could
be selected to provide a limited range in one polarity. The resistors can
be equal in value to provide an equal
voltage range at either polarity, or they can be selected to limit the
range of one polarity.

The diodes on the output of each HVPS are provided to
sink the reverse current from the HVPS of the opposite
polarity before it can reverse bias the circuitry inside the HVPS. These
diodes should be rated a minimum of
150% of the maximum voltage, i.e., 6kV diodes when used with a 4kV circuit.

The HVPS control has an initial gain error of ±1.0%. When tying
the control pins of two HVPS together and driving
them with a single signal, the error becomes ±2.0%. This error can
be minimized two ways. The first way is to
use two independent control signals from two pots or two dacs, corrected
to eliminate the error. The second way
is to use a single control signal through two 'calibration' potentiometers,
one for each HVPS. The potentiometers
can be conventional or digitally controlled. These methods are effective
whether the goal is 'factory' calibration
during initial manufacturing or "recalibration" on each system
power-up.

In low-noise systems the UltraVolt HVPS is normally ordered
with the -F Ripple Stripper® Output Filter option
and with the -M Mu-Metal shielding option. Since the output resistors isolate
the HVPS from the load with a high
impedance, this impedance can be used as an additional filter element with
the addition of a single capacitor
and resistor.